Sexual size dimorphism in birds is generally attributed to one of two
processes. Sexual selection, the more commonly invoked of the two,
favors increased size in one sex because larger individuals compete more
effectively for mates than smaller individuals (Darwin 1871).
Alternatively, ecological factors such as intersexual competition for
food (Selander 1972) or division of labor by a mated pair rearing young
(Newton 1979) might favor males and females that differ in size. In both
general cases, the advantage to sexual size dimorphism is realized by
adults. However, the advantage to adults might entail costs earlier in
life. For example, individuals of the larger sex require more energy to
attain adult size (Fiala and Congdon 1983; Slagsvold et al. 1986;
Teather 1987; Teather and Weatherhead 1988) and suffer higher rates of
starvation when food is scarce (Roskaft and Slagsvold 1985; Teather and
Weatherhead 1989). These costs to juveniles of the larger sex may
explain the consistently female-biased fledging sex ratio among species
in which males are larger (Weatherhead and Teather 1991) and may be
sufficient to limit the extent to which sexual size dimorphism can
evolve in adults (Clutton-Brock et al. 1985; Teather and Weatherhead
1989). Here we consider the consequences of the costs of sexual size
dimorphism for egg size in sexually dimorphic birds.

If sexual size dimorphism in adults results in costs to juveniles,
selection should favor any adaptation that reduces those costs. Although
the costs are borne directly by juveniles, their parents share those
costs (directly through provisioning the young and indirectly through
decreased fitness when their offspring starve), thus selection should
favor adaptations in adults that increase the survival of their
offspring. One way in which the cost of growing large could be reduced
is for nestlings to start growing at a higher proportion of their adult
size (i.e., for their mothers to lay larger eggs). Egg size should
respond to selection because, although egg size varies allometrically
with body size, substantial variation within that general relationship
exists (Gill 1990), and egg size is highly heritable (Boag and van
Noordwijk 1987). Egg size appears to influence growth (Schifferli 1980;
Howe 1976; Williams 1980; Teather 1990) and survival (Davis 1975; Howe
1976; Thomas 1983; although see Bolton [1991] concerning potentially
confounding effects of egg size and parental quality); thus by laying
larger eggs, females should lose fewer of their larger-sexed offspring
to starvation. Evidence from strongly dimorphic species suggests that
males and females hatch from eggs of similar size (Fiala 1981; Blank and
Nolan 1983; Bancroft 1984a; Weatherhead 1985; Teather 1989), thus
females do not lay larger eggs when producing the larger sex. The
alternative we test here is that females of sexually dimorphic species
in which males are larger could lay larger eggs than expected for their
body size. Because females would benefit from any reduction in mortality
of their progeny, a female trait (egg size) could easily evolve in
response to selection acting directly on a male trait (i.e., male body
size when males are the larger sex).

If egg size has been modified to reduce the cost of sexual size
dimorphism, two predictions follow. First, in species in which males are
larger, the deviation in egg size from that predicted by the
female's body size should increase as sexual dimorphism increases.
Thus, females of the most dimorphic species should lay the largest eggs
relative to their body size. Second, in species in which females are
larger than males, no relationship between egg size and sexual size
dimorphism is predicted. Rather, females should continue to lay eggs in
proportion to their body size. This follows because as females become
larger, so do their eggs, thereby automatically compensating for the
increased energetic cost of raising daughters.

Alternatively, egg size may not reflect the costs of sexual size
dimorphism, but rather egg size may simply vary with body size. Two
allometric relationships seem possible. First, egg size could vary with
female body size alone. In this case, egg-size deviations should be
independent of sexual size dimorphism. Second, egg-size deviations could
vary as some function of both male and female body size. For species in
which males are larger, this hypothesis makes the same prediction as the
cost-reduction hypothesis, namely that as size dimorphism increases,
females should lay larger eggs relative to their body size. For species
in which females are larger, however, this hypothesis predicts that
relative egg size will decrease as size dimorphism increases.

MATERIALS AND METHODS

To determine how egg size varied with sexual size dimorphism in
birds, we assembled data on adult and egg weights for species in six
taxonomic groups in which the constituent species displayed substantial
variation in sexual dimorphism. Three of the taxa (waterfowl,
shorebirds, and galliformes) have precocial young and the other three
(raptors, owls, and icterines) have altricial young. Sexual size
dimorphism is predominantly male-biased (i.e., males are larger) in the
waterfowl, galliformes, and icterines, and female-biased in the owls and
raptors, whereas biases in both directions occur among the shorebirds.
Thus, our data included substantial variation in both the patterns of
dimorphism and in nesting ecology.

We obtained weight data for males and females of 446 species from the
following sources: waterfowl--Johnsgard (1978), Palmer (1976a,b);
raptors--Palmer (1988a,b); shorebirds--Johnsgard (1981); owls--Johnsgard
(1988a), Mikkola (1983); galliformes--Johnsgard (1986, 1988b);
icterines--various sources. Values for adult and egg weights for species
not provided were obtained from Dunning (1984) and Schonwetter
(1960-1972), respectively.

By referring to different sources, we attempted to obtain the best
estimates for adult and egg sizes for each species. In several cases,
ranges rather than mean values were provided for adult weights; in such
instances we used the midpoint of this range. Where discrepancies
occurred in values between sources, or within the same source (because
of seasonal or geographic variability), final values were determined in
one of the following ways. If the mean mass for different races or
subspecies was provided, we used the one (usually nominate) for which
egg mass was reported. Where values for egg size could not be matched
with any of the races, we used mean values from all races. Similarly,
where substantial geographic variation existed, we again tried to match
the locality from which adult masses were obtained to those from which
egg masses came. Where body mass exhibited substantial seasonal
variation, we used breeding weights when available.

To test our hypotheses, we compared observed egg weights to those
expected based on female weights. The general relationship between egg
and body weight is

Two points need to be made regarding our use of Rahn et al.'s
(1975) equations for the relationship between female weight and egg
weight. The first concerns our reason for not developing our own
equations from the data we compiled. Our objective was to use the
equations to determine the expected egg weights for females of dimorphic
species were they not dimorphic. Thus, for a group such as the
icterines, in which sexual dimorphism is pronounced in most species for
which we had data, an equation based on those data alone would have been
strongly influenced by any effect that dimorphism had on egg weight
(i.e., the very phenomenon we are trying to detect). This would then
confound our objective of using residuals from the regression analysis
to determine whether dimorphism affects egg weight. By using Rahn et
al.'s general equation for passerines, for which pronounced size
dimorphism is uncommon, we avoided that problem. Less potential for
similar confounding effects existed for the other groups we analyzed,
but for consistency we used Rahn et al.'s equations throughout.

The second point concerns the use of Rahn et al.'s least-squares
linear regression (LLR) to develop their relationships between female
weight and egg weight. Because error exists in both the X and Y
variables, LLR consistently underestimates the slope of the relationship
(LaBarbera 1989). Nonetheless, LLR remains a more appropriate analysis
than reduced major axis regression (RMA) in this situation for two
reasons (Harvey and Pagel 1991). First, the [R.sup.2] values for the
equations we used were all high (0.83 to 0.96), which minimizes the
underestimate of the slope from LLR. Second, and more important, when
the objective of the regression analysis is to calculate residuals to
control for the effect of weight (as we did here), then LLR is more
appropriate, because with RMA, residuals computed parallel to the Y-axis
will be correlated with the independent variable.

Multiple species comparisons can be complicated if species share
similar attributes because of common ancestry. To reduce this problem,
we used the method suggested by Read and Weary (1990). For each
taxonomic level within our sample (e.g., species within genera, genera
within tribes, etc.), we used Spearman rank correlations to examine the
relationship between size dimorphism (expressed as the ratio male mass
to female mass) and the deviation from the egg mass expected for a given
sized female. If the deviation in egg size varied with the degree of
sexual size dimorphism, then we would expect significantly more of our
Spearman rank correlations to be the same sign (positive or negative)
than expected by chance (P [is less than] 0.05), as determined by a
binomial test. For example, 15 species of icterines yielded a total of
six comparisons (five between species of the same genera, and one among
the means of the different genera within the subfamily Icterinae), five
of which indicated a positive correlation between relative egg size and
sexual size dimorphism. Taxonomic classifications for all groups were
taken from Sibley and Ahlquist (1990). In addition, we performed single
sample t-tests on z-scores estimated from the correlation coefficients
using 0 as the population mean. Z-scores have the advantage of taking
into account the sample size used in calculating the correlation
coefficient. Thus, within genera, for example, the z-score corresponding
to a correlation coefficient from a genus with many species would be
larger than that derived from a coefficient from a genus with few
species. A significant positive mean z-score would indicate a positive
relationship between size dimorphism and relative egg weight.

Scatter plots of egg-weight deviation relative to sexual size
dimorphism illustrate two general trends. First, observed egg weights
deviated substantially from values expected based on female body weight.
Note that nearly all residuals for the blackbirds were negative,
indicating that blackbirds collectively lay relatively light eggs
compared with other passerines. Because subsequent comparisons using
these data were conducted within the blackbirds, however, it was the
relative values of the residuals that were important. Thus, the fact
that most of the values were negative had no effect on our analyses. The
second general trend was that for several taxonomic groups relative egg
weight was associated with sexual size dimorphism. This pattern was
confirmed by the hierarchical analysis. In the blackbirds and waterfowl,
in which males are heavier in dimorphic species, a significant number of
groups showed an increase in relative egg weight as size dimorphism
increased (i.e., as species became TABULAR DATA OMITTED more dimorphic,
females laid heavier eggs relative to their body weight; table 2). In
the galliformes (where males are also heavier), relative egg weight was
not significantly associated with size dimorphism, although the trend
was positive. In the genus Francolinus, the most speciose genus in our
sample of galliformes, relative TABULAR DATA OMITTED egg weight was
highly correlated with size dimorphism ([r.sub.s] = 0.658, N = 26, P [is
less than] 0.01).

In owls and raptors, in which females are heavier in dimorphic
species, we observed the opposite trend: relative egg weight decreased
as sexual dimorphism increased. However, the pattern was significant
only for the raptors.

Shorebirds were the only group that included both species with
male-biased and species with female-biased size dimorphism. When we
restricted our analysis to those species that were monomorphic or in
which males were heavier, we did not find a significant association
between relative egg weight and sexual size dimorphism. Similarly, the
association was not significant when restricted to monomorphic species
and those in which females were heavier. However, when all shorebird
species were included, a significant number of groups showed an increase
in relative egg weight as male weight increased relative to female
weight. Similarly, when all the groups in table 2 were combined,
relative egg weight increased significantly with increasing relative
male weight.

DISCUSSION

Among the groups we examined, relative egg weight varied
substantially. When examined across species, females laid relatively
heavier eggs as male weight increased relative to female weight. This
pattern was consistent with our initial hypothesis that an increase in
relative egg weight would reduce the costs for males of growing large in
species with male-biased size dimorphism. However, this pattern was also
consistent with the hypothesis that egg weight varies as an allometric
function of both male and female size. Furthermore, among species in
which females are heavier, females were found to lay proportionately
lighter eggs. Because this latter pattern was not predicted by our
cost-reduction hypothesis, we conclude that interspecific variation in
relative egg weight is best explained as a joint allometric function of
both male and female body size.

Although our results did not support our hypothesis predicting
adaptive modification of egg size in response to the costs of sexual
size dimorphism, that failure is not damaging to the underpinnings of
the hypothesis. We had predicted that females should lay heavier eggs in
species in which males are larger because heavier eggs would reduce the
cost to males of growing large. Thus, females laying heavier eggs would
have more sons survive. We found this predicted pattern between relative
egg size and size dimorphism, but it was best explained as a consequence
of allometry. However, even if our conclusion is correct regarding
allometry, males of species with male-biased dimorphism still should
survive better by hatching from heavier eggs.

Elsewhere we have explored how sex ratios (Weatherhead and Teather
1991) and nestling growth and development (Teather and Weatherhead 1994)
vary with sexual size dimorphism in birds. In both cases, we found that
although the patterns were qualitatively consistent with hypotheses that
predicted adaptations specifically in response to size dimorphism, the
patterns could be explained more parsimoniously as simple energetic or
allometric consequences of one sex becoming larger. The results we have
reported here for egg size in sexually dimorphic species appear to
provide yet a third example, in which a consequence of sexual size
dimorphism coincidentally produces an effect predicted to evolve in
response to costs associated with size dimorphism.

These results also have implications for the basis of egg-size
allometry in birds. We initially proposed two possible patterns of
allometry. Egg size could vary either with female body size alone or
with both male and female body size. We had considered the former
possibility more plausible for several reasons. Most obviously, females
produce and lay the egg. If the general allometric relationship of egg
size in birds is a consequence of the mechanics of developing,
transporting, and laying an egg of a given size, then male size should
have no influence on egg size. Among bird species, the general
allometric relationship appears to allow considerable flexibility for
both the size of eggs that birds can lay and the size of birds that can
be produced from eggs of a given size. Interspecifically, for example, a
505 g female ruddy duck (Oxyura jamaicensis) and a 1660 g female king
eider (Somateria spectabilis) both lay eggs of about 73 g.
Intraspecifically, in great-tailed grackles (Quiscalus mexicanus), 214 g
males and 119 g females are both produced from eggs of the same size.
This flexibility, coupled with high heritability for egg size, should
allow substantial scope for the adaptive modification of egg size. Thus,
we viewed egg size as essentially a female trait and did not expect male
body size to constrain the size of eggs that females lay.

Our results suggested, however, that male size influences egg size.
One might argue that this result is not unexpected because females
inherit the genetic basis for the eggs they will lay from both their
mother and father. In domestic chickens, for example, estimates of the
heritability of egg weight average approximately 0.52 whether they are
based on maternal or paternal half-sib correlations (Kinney 1969).
However, the fact that males carry genes for egg size does not protect
those genes from natural selection, because the effect of those genes
will be exposed in daughters. Thus, if selection favored females of a
particular size laying eggs of a particular size, that relationship
should evolve even though genes coding for egg size are carried by both
sexes.

Why, then, does male size exert an independent effect on egg size?
Our expectation that male body size would not have an allometric effect
on egg size was based on the assumption that the basis for egg size
allometry was a consequence of the mechanics of egg production and
transportation prior to laying. Our results suggest that genes
influencing overall body size have an epistatic effect on egg size. This
would cause egg size to evolve directly as a function of both male and
female body size and would limit the extent to which egg size could be
modified independently. The evidence from chickens is consistent with
this hypothesis (Kinney 1969). Egg weight and body weight are both
highly heritable, and the genetic correlation between egg weight and
body weight averages approximately 0.37. The fact that a female trait
(egg weight) is allometrically related to both female and male body size
suggests that the relationship between egg size and body size in
sexually dimorphic birds may provide an interesting opportunity to
investigate the basis for allometric relationships in general.

ACKNOWLEDGMENTS

We thank M. L. Forbes, S. M. Yezerinac, F. Rohwer, A. R. Palmer, and
an anonymous reviewer for their comments on the manuscript. A. F. Read
made several suggestions regarding the hierarchical analysis. Funding
for this project came from the Natural Sciences and Engineering Research
Council of Canada.